1 1 Slide Chapter 10 Project Scheduling: PERT/CPM n Project Scheduling with Known Activity Times n Project Scheduling with Uncertain Activity Times n Considering

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  • 1 1 Slide Chapter 10 Project Scheduling: PERT/CPM n Project Scheduling with Known Activity Times n Project Scheduling with Uncertain Activity Times n Considering Time-Cost Trade-Offs
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  • 2 2 Slide PERT/CPM n PERT Program Evaluation and Review Technique Program Evaluation and Review Technique Developed by U.S. Navy for Polaris missile project Developed by U.S. Navy for Polaris missile project Developed to handle uncertain activity times Developed to handle uncertain activity times n CPM Critical Path Method Critical Path Method Developed by Du Pont & Remington Rand Developed by Du Pont & Remington Rand Developed for industrial projects for which activity times generally were known Developed for industrial projects for which activity times generally were known n Todays project management software packages have combined the best features of both approaches.
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  • 3 3 Slide PERT/CPM n PERT and CPM have been used to plan, schedule, and control a wide variety of projects: R&D of new products and processes R&D of new products and processes Construction of buildings and highways Construction of buildings and highways Maintenance of large and complex equipment Maintenance of large and complex equipment Design and installation of new systems Design and installation of new systems
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  • 4 4 Slide PERT/CPM n PERT/CPM is used to plan the scheduling of individual activities that make up a project. n Projects may have as many as several thousand activities. n A complicating factor in carrying out the activities is that some activities depend on the completion of other activities before they can be started.
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  • 5 5 Slide PERT/CPM n Project managers rely on PERT/CPM to help them answer questions such as: What is the total time to complete the project? What is the total time to complete the project? What are the scheduled start and finish dates for each specific activity? What are the scheduled start and finish dates for each specific activity? Which activities are critical and must be completed exactly as scheduled to keep the project on schedule? Which activities are critical and must be completed exactly as scheduled to keep the project on schedule? How long can noncritical activities be delayed before they cause an increase in the project completion time? How long can noncritical activities be delayed before they cause an increase in the project completion time?
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  • 6 6 Slide Project Network n A project network can be constructed to model the precedence of the activities. n The nodes of the network represent the activities. n The arcs of the network reflect the precedence relationships of the activities. n A critical path for the network is a path consisting of activities with zero slack.
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  • 7 7 Slide Example: Franks Fine Floats Franks Fine Floats is in the business of building elaborate parade floats. Frank and his crew have a new float to build and want to use PERT/CPM to help them manage the project. The table on the next slide shows the activities that comprise the project. Each activitys estimated completion time (in days) and immediate predecessors are listed as well. Frank wants to know the total time to complete the project, which activities are critical, and the earliest and latest start and finish dates for each activity.
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  • 8 8 Slide Example: Franks Fine Floats Immediate Completion Immediate Completion Activity Description Predecessors Time (days) Activity Description Predecessors Time (days) A Initial Paperwork --- 3 A Initial Paperwork --- 3 B Build Body A 3 B Build Body A 3 C Build Frame A 2 C Build Frame A 2 D Finish Body B 3 D Finish Body B 3 E Finish Frame C 7 E Finish Frame C 7 F Final Paperwork B,C 3 F Final Paperwork B,C 3 G Mount Body to Frame D,E 6 G Mount Body to Frame D,E 6 H Install Skirt on Frame C 2 H Install Skirt on Frame C 2
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  • 9 9 Slide Example: Franks Fine Floats n Project Network Start Finish B 3 D 3 A 3 C 2 G 6 F 3 H 2 E 7
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  • 10 Slide Earliest Start and Finish Times n Step 1: Make a forward pass through the network as follows: For each activity i beginning at the Start node, compute: Earliest Start Time = the maximum of the earliest finish times of all activities immediately preceding activity i. (This is 0 for an activity with no predecessors.) Earliest Start Time = the maximum of the earliest finish times of all activities immediately preceding activity i. (This is 0 for an activity with no predecessors.) Earliest Finish Time = (Earliest Start Time) + (Time to complete activity i ). Earliest Finish Time = (Earliest Start Time) + (Time to complete activity i ). The project completion time is the maximum of the Earliest Finish Times at the Finish node.
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  • 11 Slide Example: Franks Fine Floats n Earliest Start and Finish Times Start Finish 3 6 B 3 6 9 D 3 0 3 A 3 3 5 C 2 12 18 G 6 6 9 F 3 5 7 H 2 5 12 E 7
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  • 12 Slide Latest Start and Finish Times n Step 2: Make a backwards pass through the network as follows: Move sequentially backwards from the Finish node to the Start node. At a given node, j, consider all activities ending at node j. For each of these activities, i, compute: Latest Finish Time = the minimum of the latest start times beginning at node j. (For node N, this is the project completion time.) Latest Finish Time = the minimum of the latest start times beginning at node j. (For node N, this is the project completion time.) Latest Start Time = (Latest Finish Time) - (Time to complete activity i ). Latest Start Time = (Latest Finish Time) - (Time to complete activity i ).
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  • 13 Slide Example: Franks Fine Floats n Latest Start and Finish Times Start Finish 3 6 6 9 B 3 9 12 D 3 0 3 A 3 3 5 C 2 12 18 G 6 6 9 15 18 F 3 5 7 16 18 H 2 5 12 E 7
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  • 14 Slide Determining the Critical Path n Step 3: Calculate the slack time for each activity by: Slack = (Latest Start) - (Earliest Start), or Slack = (Latest Start) - (Earliest Start), or = (Latest Finish) - (Earliest Finish). = (Latest Finish) - (Earliest Finish).
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  • 15 Slide Example: Franks Fine Floats n Activity Slack Time Activity ES EF LS LF Slack Activity ES EF LS LF Slack A 0 3 0 3 0 (crit.) A 0 3 0 3 0 (crit.) B 3 6 6 9 3 B 3 6 6 9 3 C 3 5 3 5 0 (crit.) C 3 5 3 5 0 (crit.) D 6 9 9 12 3 D 6 9 9 12 3 E 5 12 5 12 0 (crit.) E 5 12 5 12 0 (crit.) F 6 9 15 18 9 F 6 9 15 18 9 G 12 18 12 18 0 (crit.) G 12 18 12 18 0 (crit.) H 5 7 16 18 11 H 5 7 16 18 11
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  • 16 Slide n Determining the Critical Path A critical path is a path of activities, from the Start node to the Finish node, with 0 slack times. A critical path is a path of activities, from the Start node to the Finish node, with 0 slack times. Critical Path: A C E G Critical Path: A C E G The project completion time equals the maximum of the activities earliest finish times. The project completion time equals the maximum of the activities earliest finish times. Project Completion Time: 18 days Project Completion Time: 18 days Example: Franks Fine Floats
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  • 17 Slide Example: Franks Fine Floats n Critical Path Start Finish 3 6 6 9 B 3 9 12 D 3 0 3 A 3 3 5 C 2 12 18 G 6 6 9 15 18 F 3 5 7 16 18 H 2 5 12 E 7
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  • 18 Slide n In the three-time estimate approach, the time to complete an activity is assumed to follow a Beta distribution. n An activitys mean completion time is: t = ( a + 4 m + b )/6 t = ( a + 4 m + b )/6 n An activitys completion time variance is: 2 = (( b - a )/6) 2 2 = (( b - a )/6) 2 a = the optimistic completion time estimate a = the optimistic completion time estimate b = the pessimistic completion time estimate b = the pessimistic completion time estimate m = the most likely completion time estimate m = the most likely completion time estimate Uncertain Activity Times
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  • 19 Slide Uncertain Activity Times n In the three-time estimate approach, the critical path is determined as if the mean times for the activities were fixed times. n The overall project completion time is assumed to have a normal distribution with mean equal to the sum of the means along the critical path and variance equal to the sum of the variances along the critical path.
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  • 20 Slide Example: ABC Associates n Consider the following project: Immed. Optimistic Most Likely Pessimistic Immed. Optimistic Most Likely Pessimistic Activity Predec. Time (Hr.) Time (Hr.) Time (Hr.) Activity Predec. Time (Hr.) Time (Hr.) Time (Hr.) A -- 4 6 8 A -- 4 6 8 B -- 1 4.5 5 B -- 1 4.5 5 C A 3 3 3 C A 3 3 3 D A 4 5 6 D A 4 5 6 E A 0.5 1 1.5 E A 0.5 1 1.5 F B,C 3 4 5 F B,C 3 4 5 G B,C 1 1.5 5 G B,C 1 1.5 5 H E,F 5 6 7 H E,F 5 6 7 I E,F 2 5 8 I E,F 2 5 8 J D,H 2.5 2.75 4.5 J D,H 2.5 2.75 4.5 K G,I 3 5 7 K G,I 3 5 7
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  • 21 Slide Example: ABC Associates n Project Network 66 44 33 55 55 22 44 11 66 33 55
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  • 22 Slide Example: ABC Associates n Activity Expected Times and Variances t = ( a + 4 m + b )/6 2 = (( b - a )/6) 2 t = ( a + 4 m + b )/6 2 = (( b - a )/6) 2 Activity Expected Time Variance A 6 4/9 A 6 4/9 B 4 4/9 B 4 4/9 C 3 0 C 3 0 D 5 1/9 D 5 1/9 E 1 1/36 E 1 1/36 F 4 1/9 F 4 1/9 G 2 4/9 G 2 4/9 H 6 1/9 H 6 1/9 I 5 1 I 5 1 J 3 1/9 J 3 1/9 K 5 4/9 K 5 4/9
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  • 23 Slide Example: ABC Associates n Earliest/Latest Times and Slack Activity ES EF LS LF Slack A 0 6 0 6 0 * A 0 6 0 6 0 * B 0 4 5 9 5 B 0 4 5 9 5 C 6 9 6 9 0 * C 6 9 6 9 0 * D 6 11 15 20 9 D 6 11 15 20 9 E 6 7 12 13 6 E 6 7 12 13 6 F 9 13 9 13 0 * F 9 13 9 13 0 * G 9 11 16 18 7 G 9 11 16 18 7 H 13 19 14 20 1 H 13 19 14 20 1 I 13 18 13 18 0 * I 13 18 13 18 0 * J 19 22 20 23 1 J 19 22 20 23 1 K 18 23 18 23 0 * K 18 23 18 23 0 *
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  • 24 Slide n Determining the Critical Path A critical path is a path of activities, from the Start node to the Finish node, with 0 slack times. A critical path is a path of activities, from the Start node to the Finish node, with 0 slack times. Critical Path: A C F I K Critical Path: A C F I K The project completion time equals the maximum of the activities earliest finish times. The project completion time equals the maximum of the activities earliest finish times. Project Completion Time: 23 hours Project Completion Time: 23 hours Example: ABC Associates
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  • 25 Slide Example: ABC Associates n Critical Path (A-C-F-I-K) 66 44 33 55 55 22 44 11 66 33 55 0 6 9 13 13 18 9 11 9 11 16 18 13 19 14 20 19 22 20 23 18 23 6 7 6 7 12 13 6 9 0 4 5 9 6 11 6 11 15 20
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  • 26 Slide Example: ABC Associates n Probability the project will be completed within 24 hrs 2 = 2 A + 2 C + 2 F + 2 H + 2 K = 4/9 + 0 + 1/9 + 1 + 4/9 = 4/9 + 0 + 1/9 + 1 + 4/9 = 2 = 2 = 1.414 = 1.414 z = (24 - 23)/ (24-23)/1.414 =.71 z = (24 - 23)/ (24-23)/1.414 =.71 From the Standard Normal Distribution table: From the Standard Normal Distribution table: P(z